1. Field of the Invention
The present invention relates to a localized-field static magnetizing device. It is applicable in particular to arrangements for reading coded magnetic information such as check-reading arrangements.
2. Description of the Prior Art
It is known that present day data-processing systems often include data infeed arrangements which make use of slips bearing coded magnetic information. These slips may for example be bank checks, giro checks or credit cards.
The information generally consists of a succession of alpha-numeric characters printed on the slips, that is to say a succession of letters of the alphabet, figures, punctuation marks, etc., which indicate, in the case where the slip is a check for example, the number of the check or the account number of the drawer of the check.
Each character is formed by a set of bars composed of magnetic ink. The number of bars, the distance between the bars, and their relative disposition are unique to each character and are coded in accordance with known codes such as the CMC7 code for example.
An example will be considered of a check and a corresponding data-infeed device which is termed "a check reader". The check reader converts the coded magnetic information given by the characters printed on the check into a succession of electrical signals. It is connected to electronic shaping circuits which convert this succession of electrical signals into a succession of square-wave electrical pulses which are transmitted to electronic circuits for recognizing the characters printed on the check. As soon as the characters corresponding to this succession of square-wave electrical pulses have been identified, it is possible, by means of a calculating unit in the data-processing system at which the check reader forms a part, to cause operations relating to the check to be performed such as debit or credit operations, the operation of updating the account of the drawer, and so on.
So that the subject of the invention may be better understood, it is necessary to review the following facts about magnetism.
To magnetize a magnetic material, the material is first subjected to a positive magnetic field whose intensity is sufficient to saturate the material, that is to say for the magnetic induction in the material to reach a limiting value B.sub.s. The magnetic field is then reduced to zero. There then remains a magnetic induction of a level other than zero (+Mr) termed the residual magnetic induction, which is characteristic of the material. In other words, magnetizing a magnetic material amounts to saturating it magnetically.
It will be also recalled that if a negative magnetic field is then applied to this material, the magnetic induction in the material will fall to zero at a value of H termed the coercive field value H.sub.c. The curve representing the change in the magnetic induction as a function of the field H is termed a hysteresis loop.
It will further be recalled that a magnetic material which has been magnetized creates in the immediate vicinity on its surface a magnetic leakage field which can be shown to be a function of the coercive field H.sub.c and which is theoretically at most equal to the latter. In practice the leakage field is always less than the coercive field.
Check readers generally include:
a magnetizing device to magnetize the bars making up the characters printed on the check.
a magnetic transducer device sensitive to the magnetic leakage field created by the bars which have been magnetized by the magnetizing device, which emits, in response to this magnetic leakage field, an electrical signal which is transmitted to the aforementioned electronic shaping circuits.
The check is moved by a mechanical check transporting device and its position in the latter is such that all the bars forming the characters pass in succession across the front of the magnetizing device and the magnetic transducer device in close proximity thereto. It is in fact necessary to magnetize the bars before passing them in front of the magnetic transducer device since, when the characters are printed on the check, either the induction in the bars is zero or else the level and sense of the magnetic induction within the bars vary from one bar to the next. It will be appreciated that the object of the magnetizing device is to cause the level and sense of the magnetic induction to be identical in all the bars, so that it is equal to the residual induction of the magnetic ink from which they are formed.
In present day pratice, magnetizing devices are formed by a magnetic head made up of a magnetic core around which is wound a winding and which is provided with a wide air gap. The bars pass close to the air gap at a very small distance therefrom. A constant current is caused to flow in the windings and this creates a magnetic field in the magnetic core. That proportion of the lines of force of the magnetic field which close outside the air gap on a level therewith and which represent the magnetic leakage, cause the bars to become magnetized provided that the amperage of the current in the winding is sufficiently high. It will be appreciated that under these conditions the electro-magnetic efficiency of the magnetic head is very low: in effect for a high level of electrical energy applied to the winding, a low level of magnetizing energy is obtained. In addition, such a magnetic head has to be carefully manufactured and is relatively expensive and bulky.
Magnetizing devices are known which overcome these disadvantages. They are formed by a magnetic layer having a high coercive field which is deposited on a nonmagnetic substrate. Layers referred to as having a high coercive field are ones whose coercive field is at least equal to that of the bars.
A layer of this kind creates in the immediate vicinity of its surface a high and restricted magnetic leakage field, (of a few hundred oersteds). It is for this reason that the bars forming the characters on the check are made to pass at a very short distance from the magnetic layer (several tens of microns).
Generally speaking, the magnetic induction created within the bars by the magnetic leakage field of the layer, although considerable, is still less than the saturation induction B.sub.s of the magnetic ink. The result is that the magnetic induction which remains within the bars after they have passed in front of the magnetic layer is less than the residual induction of the magnetic ink. It is clear than, in theory, only layers whose magnetic leakage field is extremely high (of the order of 4 to 5 times the coercive field of the bars) at the distance indicated above, would enable a magnetic induction which is as close as possible to the saturation induction B.sub.s, but which still remains lower than it, to be created within the bars. Such layers which need to have a very high coercive field are technically difficult to produce and their cost is high.